I. Field
The present invention relates generally to communication, and more specifically to techniques for transmitting data on multiple parallel channels in a multiple-input multiple-output (MIMO) communication system.
II. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission and is denoted as an (NT, NR) system. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS spatial channels, where NS≦min {NT, NR}, as described below. Ns data streams may be transmitted on the NS spatial channels. The MIMO system can provide increased transmission capacity if the NS spatial channels created by the multiple transmit and receive antennas are used for data transmission.
A major challenge in a MIMO system is selecting suitable rates for data transmission on the MIMO channel based on channel conditions. A “rate” may indicate a particular data rate or information bit rate, a particular coding scheme, a particular modulation scheme, a particular data packet size, and so on. The goal of the rate selection is to maximize the overall throughput on the NS spatial channels while meeting certain quality objectives, which may be quantified by a target packet error rate (e.g., 1% PER) or some other measures.
The transmission capacity of each spatial channel is dependent on the signal-to-noise-and-interference ratio (SINR) achieved by that spatial channel. The SINRs for the NS spatial channels are dependent on the channel conditions and may further be dependent on the manner in which the data streams are recovered at the receiver. In one conventional MIMO system, a transmitter encodes, modulates, and transmits each data stream in accordance with a rate selected based on a model of a static MIMO channel. Good performance can be achieved if the model is accurate and if the MIMO channel is relatively static (i.e., does not change much over time). In another conventional MIMO system, a receiver estimates the MIMO channel, selects a suitable rate for each spatial channel based on the channel estimates, and sends Ns selected rates for the Ns spatial channels to the transmitter. The transmitter then processes Ns data streams in accordance with the selected rates and transmits these streams on the Ns spatial channels. The performance of this system is dependent on the nature of the MIMO channel and the accuracy of the channel estimates.
For both conventional MIMO systems described above, the transmitter processes and transmits each data packet for each spatial channel at the rate selected for that spatial channel. The receiver decodes each data packet received on each spatial channel and determines whether the packet is decoded correctly or in error. The receiver may send back an acknowledgment (ACK) if the packet is decoded correctly or a negative acknowledgment (NAK) if the packet is decoded in error. The transmitter may retransmit each data packet decoded in error by the receiver, in its entirety, upon receiving a NAK for the packet.
The performance of both MIMO systems described above is highly dependent on the accuracy of the rate selection. If the selected rates for the spatial channels are too conservative (e.g., because the actual SINRs are much better than the SINR estimates), then excessive system resources are expended to transmit data packets and channel capacity is underutilized. Conversely, if the selected rates for the spatial channels are too aggressive, then the data packets may be decoded in error by the receiver and system resources may be expended to retransmit these packets. Rate selection for a MIMO system is challenging because of (1) greater complexity in the channel estimation for a MIMO channel, (2) the time-varying and independent nature of the spatial channels, and (3) interaction among the multiple data streams transmitted on the spatial channels.
There is therefore a need in the art for techniques to efficiently transmit data on multiple spatial channels in a MIMO system and which do not require accurate rate selection in order to achieve good performance.